The present invention relates to an apparatus and method employing a checkered photodetector array employing PIN photodiodes for detecting the location of a centroid of a spot produced by impinging electromagnetic radiation.
Optical sensing techniques are used to determine the position, dimensions, attitude and angular displacement of a moving object. They are also used to track and/or sense the motion of a mechanical part belonging to a larger system or perform other position measurements requiring high levels of accuracy. Among other, these techniques find numerous applications in the fields of robotics, artificial vision, mechanical control and feedback. For an example of a prior art method and apparatus for electro-optically determining the dimension, location and attitude of objects the reader is referred to U.S. Pat. No. 6,211,506 to Pryor et al.
Most of the sensing techniques use a coherent radiation source to generate a beam of electromagnetic radiation in a wavelength range suitable for the particular environment and application. For example, the source can be a laser delivering a beam of light in the visible wavelength range. The light beam is reflected from the object whose position is to be sensed to a position sensitive photodetector (PSD). The reflected light impinges on the PSD and produces a spot whose two-dimensional extent is analyzed to find the centroid.
Locating the centroid of a light spot presents a number of challenges due to ever-increasing requirements for high sensitivity, high resolution, linearity of response, immunity to stray light and speed. PSDs are generally divided into two groups: continuous response position sensitive detectors (CRPSD) and discrete response position sensitive detectors (DRPSD). A CRPSD is a detector that determines/calculates the centroid of a light distribution that may include stray light components in addition to a desired light spot. A DRPSD is a detector that samples and analyzes the entire light distribution to determine the position of the desired light spot within the light distribution.
CRPSDs typically use lateral effect photodiodes (LEPs) and geometrically shaped photo-diodes (wedges or segmented) such as described by A. Makynen and J. Kostamovaara, xe2x80x9cLinear and sensitive CMOS position-sensitive photodetectorxe2x80x9d, Electronics Letters, Vol. 34, No. 12, pp. 1255-56 (Jun., 11, 1998) and in A. Makynen et al., xe2x80x9cHigh accuracy CMOS position-sensitive photodetector (PSD)xe2x80x9d, Electronics Letters, Vol. 33, No. 2, pp. 128-130 (Jan. 16, 1997). The first of these references takes note of the nonlinearity and high noise suffered by LEPs in practical applications despite their large-area continuous construction and proposes a CMOS-compatible PDS using phototransistors to achieve higher resolution, accuracy and linearity. The phototransistors are small-sized and arranged to form a dimensionally accurate array, in which the emitters of every other phototransistor in each row are connected to the row current line and every other to the column current line. The photocurrents are processed using two separate arrays of polysilicon resistors with homogenous resistivity. The use of such array of phototransistors improves resolution in comparison to a conventional LEP and achieves good linearity. The second of these references describes further improvements to a CMOS PSD having a similar array construction to render it optimal for outdoor use.
A further review and teaching of multi-pixel PSDs using CMOS technology is found in Davies W. DeLimaMonteiro, et al., xe2x80x9cVarious Layouts of Analog CMOS Optical Position-Sensitive Detectorsxe2x80x9d, SPIE Conference on Electronics for Solid State Sensors, SPIE Vol. 3794, pp. 134 (July 1999). This reference teaches the use of CMOS technologies to produce several array geometries and interconnections including an array of photodiodes in a chessboard-like structure.
DRPSD are generally implemented using an array of photosensors that are read out serially by metal oxide semiconductor field effect transistor (MOSFET) switches or a charge coupled device (CCD) as disclosed by F. Blais and M. Rioux, xe2x80x9cReal-Time Numerical Peak Detectorxe2x80x9d, Signal Processing, Vol. 11, No. 2, pp. 145-155 (1986). Since a DRPS samples the entire distribution, it can typically achieve higher accuracy levels than CRPSD, but at a slower speed relative to a CRPSD.
U.S. Pat. No. 6,297,488 to Beraldin et al. teaches a position sensitive light spot detector developed to improve the resolution and speed of a PSD. This detector includes a CRPSD (e.g., a lateral effect photodiode) for determining a first centroid of the light distribution and a DRPSD (e.g., a multiplexed array) for determining a second centroid of the light distribution within a reading window about the first centroid and within the light distribution. The second centroid represents the position of the light spot in the light distribution. Beraldin""s multiple stage approach exploits the high resolution and speed offered by CRPSDs with the accuracy under variable lighting conditions offered by traditional DRPSDs.
The optical PSDs taught by the prior art can be used in many applications where remote, touch-free sensing and extremely high sensitivity are required. Some of these applications take advantage of a geometric leveraging effect to monitor mechanical devices. In accordance with this effect, the light beam is allowed to travel a large displacement across the face of the PSD in response to a small movement of the mechanical device being monitored. This approach allows the user to increase measurement sensitivity and decrease sensitivity to misalignments between the remote PSD and the mechanical device.
However, a high level of geometric leveraging creates a need for a large PSD. In addition, certain applications require that a large number of mechanical devices be monitored at the same time. Using a dedicated sensor for each device is extremely costly and not feasible or downright impossible due to physical constraints. It would be advantageous to resolve this problem by providing a single, large PSD having a sufficiently large bandwidth to sense a large number of multiplexed beams.
Unfortunately, the prior art technologies cannot be used for developing a large PSD with a sufficient bandwidth for monitoring a large number of parts. Specifically, in applications requiring a PSD as large as 50 mmxc3x9750 mm and a sampling rate of 25 MHz, even photodetectors built with CMOS are no longer fast enough. Thus, it would be a major improvement in the art to provide a single apparatus for monitoring the position of a large number of objects or mechanical parts while taking advantage of a high degree of geometric leveraging.
In view of the shortcomings of the prior art, it is a primary object of the present invention to provide an apparatus for monitoring a large number of objects or mechanical parts simultaneously. The apparatus is to have a sufficiently large detection area, e.g., 50 mmxc3x9750 mm or more, to permit a high level of geometric leveraging. In particular, the apparatus should have sufficient bandwidth to track the centroids of spots produced by beams reflected from as many as 25 million objects or mechanical parts each second (25 MHz bandwidth). These and other objects and advantages will become apparent upon reading the following description.
The objects and advantages are achieved by an apparatus for detecting a centroid of a spot produced by electromagnetic radiation, most commonly optic radiation in the visible wavelength range. In contrast to prior art devices, the present apparatus has an array of PIN photodiodes serving as photodetectors. The PIN photodiodes are organized in columns and in rows. Vertical connections are used to interconnect the PIN photodiodes in the columns in accordance with a first pattern that interconnects two or more adjacent columns. Horizontal connections are used to interconnect PIN photodiodes in the rows in accordance with a second pattern that interconnects two or more adjacent rows. The interconnections are made such that there are no anode connections between the PIN photodiodes in the rows and columns. The apparatus has a processing circuitry for deriving an X-extent of the centroid from current signals obtained from the columns and a Y-extent of the centroid from current signals obtained from the rows. In the preferred embodiment, the first and second patterns of interconnections preferably include just two adjacent columns and two adjacent rows, respectively and form a checkerboard interconnect pattern.
The apparatus operates on the principle that by interconnecting adjacent rows and columns of PIN photodiodes, e.g., in a checkerboard pattern, the resolution of the array is reduced only slightly but the processing electronics is reduced by half. That is because two interconnected adjacent columns yield only one current signal and two interconnected adjacent rows also yield just one current signal. Therefore, rather than four signals (two column signals and two row signals) only two current signals need to be analyzed by the processing circuitry.
The processing circuitry is equipped with appropriate multiplexing circuit for multiplexing a certain number of columns and rows. The processing circuitry has a discrimination circuit for selecting which rows and which columns should be examined based on the illumination level. Thus, for example, rows and columns registering negligible current signals and thus corresponding to a very low illumination level can be left out of consideration by the discrimination circuit.
In this embodiment, or in another embodiment, the processing circuitry is equipped with a logic for initially measuring the X- and Y-extents from all columns and rows, and measuring the X-coordinate and Y-coordinate of the centroid from only a selected number of columns and rows. Such initial measurement can be repeated periodically to ensure that only rows and columns, which yield current signals above a certain threshold level, are examined and used for deriving the X- and Y-coordinates of the centroid. The processing steps involved are performed by the processing circuitry and use of appropriate biasing.
In any of the above-mentioned embodiments, or in a still different embodiment the processing circuitry also has a calibration device for adjusting at least one detection characteristic of the PIN photodiodes. Dark leakage currents, forward voltage drops and other factors will typically condition the detection characteristics of the PIN photodiodes. It is also convenient to equip the apparatus with a filtering device such as a time-domain cross-talk filter or a weighted average noise compression filter to reduce the effects of noise on the determination of centroid position or X- and Y-extents.
In another embodiment or in any of the above-mentioned embodiments, the processing circuit can also be provided with a self-test circuit.
The arrangement of PIN photodiodes in the array requires a novel structure, wherein all the anode connections are made from one side and all the PIN photodiodes share one common cathode. The common cathode is provided on the backside of the apparatus facing the impinging electromagnetic radiation. The anode connections are made on the front side opposite the common cathode. A top cathode is also provided on the front side of the apparatus. Preferably, the top cathode is in the form of cathode rings surrounding the individual PIN diodes. The presence of the cathode rings will prevent undesirable inversion effects between the PIN photodiodes and collect the cathode current signal with as little series resistance as possible to ensure rapid operation.
The details of the apparatus and method of invention are explained in the detailed description with reference to the attached drawing figures.